Cystic fibrosis (CF) is the most common serious/lethal chronic single- gene mediated inherited disease among Causasians. Remarkable increases in life expectancy for CF patients have stemmed from improved treatment and prevention of severe pulmonary infections, especially with Psuedomonas species. As a result of the overall aging of the entire population of people with DF, medical deseases and complicatons of treatments that were previously not seen in the younger patients are now becoming more prevalent and clinically important. One medical problem that may become increasingly important for patients with CF is osteoporosis. A number of factors may put patients with CF at increased risk for early osteoporosis including intermittent glucocorticoid use, intestinal malabsorption with possible vitamin D and calcium deficiency, and increased bedrest and sedentary life style. Previous work has demonstrated that patients with CF may have a significantly higher prevalence of osteoporosis than age and sex-matched healthy controls. However, the preliminary studies involved small sample sizes, non-random and biased patient groups and did not incorporate all of the measurements of current scientific interest for osteoporosis in these patients. A comprehensive, longitudinal study, using state of the art techniques and measurements, of osteoporosis in a ramdonly enrolled group of adult patients with CF is needed. The specific aim of this study is to determine the prevalence, extent and associated clincial and biochemical markers of reduced bone density in patients with CF. Additionally, this will be the first study to investigate the relationship between bone density and specific cystic fibrosis transmembrane regulator (CFTR) genotypes as well as the rate of progression of osteoporosis in patients with CF.